dynamic link interface
char *path, int
*handle, const char
void *addr, Dl_info
*handle, int cmd,
These functions provide an interface to the run-time linker ld.so(1). They allow new shared objects to be loaded into a process's address space under program control.
function takes the name of a shared object as its first argument. The shared
object is mapped into the address space, relocated, and its external
references are resolved in the same way as is done with the implicitly
loaded shared libraries at program startup.
The path parameter can be specified as either an absolute pathname to a shared library or just the name of the shared library itself. When an absolute pathname is specified, only the path provided will be searched for the shared library. When just a shared library is specified, the same paths will be searched that are used for “intrinsic” shared library searches.
Shared libraries take the following form:
When a shared library is specified without a version or with a
partial version, the same library search rules apply that are used for
“intrinsic” shared library searches. A null pointer supplied
for path will return a special
handle that behaves the same as the
The mode parameter specifies symbol resolution time and symbol visibility. One of the following values may be used to specify symbol resolution time:
One of the following values may be used to specify symbol visibility:
To specify both resolution time and visibility, bitwise inclusive OR one of each of the above values together. If an object was opened with RTLD_LOCAL and later opened with RTLD_GLOBAL, then it is promoted to RTLD_GLOBAL.
Additionally, the following flag may be ORed into the mode argument:
The main executable's symbols are normally invisible
symbol resolution. Those symbols will be visible if linking is done with
-rdynamic, which is equivalent to
All shared objects loaded at program startup are globally visible.
returns a handle to be used in calls to
dlctl(). If the named shared object has already been
loaded by a previous call to
dlopen() and not yet
handle referring to the resident copy is returned.
unlinks and removes the object referred to by handle
from the process address space. If multiple calls to
dlopen() have been done on this object or the object
is a dependency of another object then the object is removed when its
reference count drops to zero.
dlclose() returns 0
on success and non-zero on failure.
searches for a definition of symbol in the object
designated by handle and all shared objects that it
depends on. The symbol's address is returned. If the symbol cannot be
NULL is returned.
may also be called with special handles.
dlsym() respects symbol visibility as specified by
dlopen() mode parameter.
However, the symbols of an object's dependencies are always visible to it.
The following special handles may be used with
- Interpreted as a reference to the executable or shared object from which
the call is being made. Thus an object can reference its own symbols and
the symbols of its dependencies without calling
- All the visible shared objects and the executable will be searched in the order they were loaded.
- The search for symbol is limited to the visible
shared objects which were loaded after the one issuing the call to
dlsym(). Thus, if
dlsym() is called from the main program, all the visible shared libraries are searched. If it is called from a shared library, all subsequently visible shared libraries are searched.
- The search for symbol is limited to the shared
object issuing the call to
dlsym() and those shared objects which were loaded after it that are visible.
queries the dynamic linker for information about the shared object
containing the address addr. The information is
returned in the structure specified by info. The
structure contains at least the following members:
const char *dli_fname
- The pathname of the shared object containing the address addr.
- The base address at which the shared object is mapped into the address space of the calling process.
const char *dli_sname
- The name of the nearest run-time symbol with an address less than or equal
If no symbol with a suitable address is found, both this field and dli_saddr are set to
- The address of the symbol returned in dli_sname.
If a mapped shared object containing
addr cannot be found,
returns 0. In that case, a message detailing the failure can be retrieved by
dlerror(). On success, a non-zero value is
returned. Note: both strings pointed at by dli_fname
and dli_sname reside in memory private to the run-time
linker module and should not be modified by the caller.
In dynamically linked programs, the address of a global function will point to its program linkage table entry, rather than to the entry point of the function itself. This causes most global functions to appear to be defined within the main executable, rather than in the shared libraries where the actual code resides.
provides an interface similar to
control several aspects of the run-time linker's operation. This interface
is currently under development.
returns a character string representing the most recent error that has
occurred while processing one of the other functions described here. If no
dynamic linking errors have occurred since the last invocation of
NULL. Thus, invoking
dlerror() a second time, immediately following a
prior invocation, will result in
ld(1), ld.so(1), elf(5)
Some of the
dl* functions first appeared
in SunOS 4.
Loading untrustworthy libraries into the process's address space
dlopen is very dangerous because
system-dependent initialization steps occur including the calling of
constructor functions, even if the library is otherwise unused.